Mobile communication systems use various systems to detect a digital modulated signal. In general, a so-called delay detection system is used since it has a good burst frame efficiency in comparison with heterodyne synchronous detection system and envelop detection system and has a good error ratio characteristic under Rayleigh fading.
in the delay detection system, a digital modulated signal is received as an input signal, and then the input signal is converted into a signal representing a phase change (hereinafter referred to as `instantaneous phase signal`) before, e.g., a demodulator, and then the instantaneous phase signal is input to the demodulator. The demodulator outputs a delayed instantaneous phase signal with a delay of one symbol, and then the instantaneous phase signal and delayed instantaneous phase signal are input to a subtracter to obtain a difference for delay detection, and then the difference is input to a determination circuit to conduct the determination of data, when a delayed instantaneous phase signal to be input one symbol before and an instantaneous phase signal to be input at this time are coincident with each other, a negative value is output, and, when they are not coincident with each other, a positive value is output, and then two-bit data according to the phase of symbol are output. In particular, the delay detection system is suitably used for the demodulation on the receiver side of a communication that is modulated by using DQPSK(differential quadrature phase shift keying).
On the other hand, a demodulation system using error diffusion type delay detection is disclosed in Japanese patent application laid-open No. 8-32640(1996), which describes that, when a received signal with a level sufficiently higher than that of a noise to be superposed on the communication path etc., the data error ratio can be highly improved.
In a base station for mobile stations, when an input signal with a level higher than a predetermined level is transmitted from the mobile stations, a sufficiently accurate data detection can be conducted by using the delay detection system of error diffusion type. However, with the reduction in the level of input signal, the improvement of data error ratio is lowered. When the signal level is further reduced and the superposed noise is increased, the data error ratio may be, on the contrary, deteriorated.
In particular, in case of the combination diversity system with a large diversity effect which has inputs through several antennas, the input signal level to each of the receiving systems when a certain error ratio is obtained is low. Therefore, when the delay detection system of error diffusion type is applied before the operation of combining the input signals from the receiving systems, the improvement of data error ratio may not be obtained at a weak electric field level.
Thus, when the ratio of an input signal level to a noise level superposed on the received signal is low, a receiver conducting the combination diversity cannot obtain a sufficient improvement effect of error ratio even by using the delay detection of error diffusion type.